An FPD formed of a pair of glass substrates bonded to one another is produced by mutually bonding two large-sized mother glasses and then breaking the glasses into a predetermined size. When the bonded mother glasses are broken, in a scribe step having a scribing device, scribe lines are previously formed with a cutter on the respective mother glasses, and the mother glasses with the scribe lines formed thereon are conveyed to a break step where vertical cracks immediately under the scribe lines are penetrated (broken) in a thickness direction of the mother glasses. More specifically, the step of breaking a mother substrate is constituted of the scribe step and the break step provided after the scribe step.
In recent years, a method of using a laser beam for formation of a scribe line has come into practical use in the scribe step, as disclosed in Japanese Patent No. 3027768. In a method for forming a scribe line on a glass substrate by the use of a laser beam, as shown in FIG. 1, a glass substrate 111 is irradiated with a laser beam from a laser oscillation device 112. The laser beam emitted from the laser oscillation device 112 forms an elliptical laser spot LS on the glass substrate 111 along a line to be scribed. The glass substrate 111 and the laser beam emitted from the laser oscillation device 112 are relatively shifted along a longitudinal direction of the laser spot.
Moreover, a cooling medium, such as cooling water, is sprayed from a cooling nozzle 113 in the vicinity of the laser-beam irradiation region on a surface of the glass substrate 111, so as to form a scribe line. On the surface of the glass substrate 111 irradiated with the laser beam, compression stress generates due to heating by the laser beam and, thereafter, tensile stress generates due to the spraying of the cooling medium. Therefore, since the tensile stress generates in the region in the vicinity of the region where compression stress has generated, a stress gradient occurs between the two regions according to the respective stresses, and for generating a blind crack (vertical crack) formed at an end of the glass substrate 111, a blind crack (vertical crack) is formed from a cutout (trigger) along the line to be scribed on the glass substrate 111.
Since the vertical crack, formed on the surface of the glass substrate 111 by the use of the laser beam in the manner as described above, is minute and thus normally invisible to the naked eye, the crack is called a blind crack BC.
After the blind crack (vertical crack) BC as the scribe line is formed on the glass substrate 111, the glass substrate 111 is conveyed to the subsequent break step, where a force is applied onto the glass substrate such that a bending moment acts in a width direction of the blind crack BC, to allow the vertical crack to penetrate in the thickness direction of the glass substrate, and the glass substrate 111 is thereby broken along the blind crack BC as the scribe line.
There are some cases where a blind crack BC is not normally formed with the above-described scribing device when conditions are not appropriately set, the conditions including heating by irradiation energy per unit area of a laser beam, cooling by a cooling medium, and a relative shifting speed of the laser beam and the glass substrate. If the glass substrate on which the blind crack BC has not been normally formed is supplied to the subsequent break step, the glass substrate is not broken along the blind crack BC, which may lead to damage of the glass substrate. When damaged in the break step, the glass substrate cannot be used as a component of an FPD, resulting in loss of economical efficiency as well as deteriorated production efficiency of the FPD. Further, the damage of the glass substrate might cause damage of a device for breaking a glass substrate itself.
It is therefore desirable to check that the blind crack BC has been certainly formed on the glass substrate in the scribing device. However, since the blind crack BC to be formed on the glass substrate is minute, it normally cannot be visually inspected nor directly observed with a CCD camera. Hence, there has been a problem in that normal formation of a blind crack (vertical crack) BC cannot be readily checked in the scribing device.
Further, since there is a possibility that a laser beam emitted from a laser may affect the eye, it is problematic in terms of the safety to visually check the formation of the blind crack under irradiation of the laser beam during scribing.
The present invention solves the above-mentioned problems, and aims to provide a device and a method for scribing a brittle material substrate, which are capable of certainly checking a formation state of a blind crack to be formed on a surface of a brittle material substrate such as a glass substrate to certainly break the brittle material substrate in a step of breaking a brittle material substrate, and also provide an automated breaking line using the device and the method.